1. Field of the Invention
The present invention relates to a delayed coking process and, more particularly, to a delayed coking process that minimizes the production of petroleum coke derived from petroleum residua. More particularly, the present invention relates to a delayed coking process incorporating a method of preparing a delayed coker feed charge that minimizes coke make and maximizes the make of more valuable liquid products.
2. Description of the Prior Art
As the availability of lighter crude oil sources diminishes, refiners are being forced to deal with heavier crude oil feedstocks. This comes at a time when exploring for oil and gas formations is becoming increasingly more expensive and there is an increasing demand for refined products, particularly transportation fuels, such as gasoline and diesel fuel. At the same time, the markets for heavy fuel oils is diminishing. Accordingly, refiners are faced with the necessity for finding conversion processes to convert the heavy crude oil feedstocks and the various petroleum residua (residua) that occur in the normal refining processes to more useful and profitable lighter products while minimizing the production of heavy fuel oils and coke.
Existing processes for converting heavy crude feedstocks and residua to useful, lighter products include fluid catalytic cracking (FCC), residue catalytic hydrocracking (HC), and thermal cracking, such as visbreaking, delayed coking, and fluidized bed coking. Although not technically a conversion process, solvent deasphalting of residua is also becoming popular to produce feedstocks for the above-mentioned conversion processes.
The catalytic conversion processes all possess high conversion capabilities and allow for flexibility in the yield structures but are saddled with high operating costs, occasioned by expensive catalysts and/or reactions that take place at high temperatures and pressures, necessitating the use of expensive equipment. Of the thermal conversion processes noted, visbreaking has somewhat limited conversion capabilities, the conversion being limited to some extent by the end use of the resulting visbreaker tar. The visbreaker tar may also exhibit instability and incompatibility when mixed with other hydrocarbon materials. The delayed coking process is used to maximize production of liquid products while typically producing a low quality/low value coke that is used as a solid fuel. Ideally, when producing fuel grade coke in a delayed coking operation, the objective is to maximize conversion to liquid products and minimize production of fuel grade coke. While high coke yield is desirable for the production of high quality/high value needle coke and coke for anode manufacturing used in the metallurgical industries, manufacturing of fuel grade coke is to some extent considered a last resort in an attempt to extract maximum value from the crude oil.
In typical refinery processes, there are produced bottoms or residue fractions, referred to herein as "petroleum residuum" or "petroleum residua." For example, low value petroleum residuum, known as VTB, forms the bottoms fraction from a vacuum distillation tower, such towers generally being used to further fractionate virgin atmospheric-reduced crude oil. Typically, the VTB from such vacuum distillation columns generally include all the material boiling above a selected temperature, usually at least 480.degree. C. and often as high as 590.degree. C. Petroleum residua have typically presented serious, economic disposal problems, as it has been difficult to convert the streams to more valuable products in an economic manner. Generally speaking, petroleum residua contain components of large molecular size and weight and are generally characterized by three specific ingredients: (a) asphaltenes and other high molecular weight aromatic structures that inhibit the rate of hydrotreating/hydrocracking and cause catalyst deactivation; (b) metal contaminants that occur naturally in the crude oil or result from prior treatment of the crude oil, which contaminants deactivate hydrodesulfurization and cracking catalysts and interfere with catalyst regeneration; and (c) a relatively large content of sulfur and nitrogen compounds that give rise to objectionable quantities of SO.sub.2, SO.sub.3, and NO.sub.x upon combustion of the petroleum residuum. In addition, nitrogen compounds deactivate hydrotreating/hydrocracking catalysts. Thus, these residua pose economic problems if catalytic processes are used for their conversion to lighter, more valuable components.